The present invention relates generally to apparatus used for reception and transmission of radio frequency signals. More particularly, the present invention relates to apparatus which are used to selectively add and amplify differential signals while maintaining control over the output gain.
The demand for more sophisticated communications equipment has increased exponentially in response to the popularity of cellular telephones, personal computers, and the internet. To keep up with this demand, mixed-signal (analog and digital) designs have become more prevalent in the manufacture of integrated circuits.
Several problems arise in such designs, one of the most difficult being the hostile environment created by the relatively large transitions of digital signals between the voltage supply and ground rails at rates which are usually considered to be in the realm of radio frequencies. Attempting to simultaneously receive and process relatively weak radio-frequency (RF) signals within the confines of a typical desktop personal computer, or using Personal Digital Assistant (PDA) chassis, for example, may thus present a very difficult design challenge. This is readily apparent when consideration is given to the fact that current high-end microprocessor clock speeds exceed 1 Ghz, while cellular telephones and other wireless devices commonly operate in the frequency range of 0.9 to 1.8 Ghz.
Differential amplifier circuits, which receive two input signals and provide an output signal which is a function of the difference between the input signals, are sometimes used to reduce the effects of noise on desired signals. As an example, one type of differential amplifier in the prior art includes first and second n-channel field effect transistors (NFET transistors), referred to as a differential pair. The input signals, Vin+and Vinxe2x88x92, are received at the gates of the first and second nFET transistors, respectively. If first and second loads are connected between the drains of the first and second nFET transistors, respectively, and a single power supply, the output signals Vout+and Voutxe2x88x92will appear at the drains of the first and second NFET transistors, respectively. The sources of the first and second nFET transistors are typically coupled to ground through a third nFET transistor, which serves as a source of bias current.
If the first and second loads are matched, and the first and second nFET transistors are matched, the difference between Vout+and Voutxe2x88x92is a function of the difference between Vin+and Vinxe2x88x92. In this way, the differential amplifier suppresses or rejects voltages which are included in both input signals. Examples of suppressed voltages include a DC offset voltage and noise appearing on both input signals. This suppression is referred to as common mode rejection, and can be a very useful way of increasing the effectiveness of weak signal reception. Thus differential topologies are highly desired for use within mixed-signal designs.
Universal RF front-end design often requires the use of in-phase and quadrature components for both transmitted and received signals. These signals are often mixed and added at different stages of the signal reception and transmission process, for example, to generate various types of modulation (e.g., amplitude modulation, frequency modulation, phase modulation, frequency-shift keying, phase-shift keying, etc.). As noted above, differential signal processing within RF circuitry provides a way of producing more robust designs, giving increased protection from common mode noise coupled in from nearby, purely-digital, circuitry. When multiple circuit modules are cascaded in a signal processing chain, it is also desirable to make use of automatic gain control (AGC) as a way of conveniently adjusting the output of such xe2x80x9cbuilding blocksxe2x80x9d or modules within a particular RF design, such as a transceiver, so that subsequent amplification stages are not over-driven.
Thus, there is a need in the art for apparatus which foster the use of differential signal processing within the fundamental modules used in mixed-signal RF circuit design, such as mixers, adders, and amplifiers. Such apparatus should also provide RF designers with the ability to use AGC, along with other techniques which can be used to decrease the strength of unwanted (out-of-band) signals before they enter the signal processing chain. Such apparatus are most useful in furnishing the type of analog RF circuit design tools capable of functioning effectively within the hostile signal environment present in sophisticated modern mixed-signal communications equipment.